Automated Pyrometer Tracking in a Spark Plasma Sintering Apparatus and Method

ABSTRACT

A system and method for compressively sintering a material is described. The method comprises the steps of loading a material to be sintered into a die cavity of a die set and placing the die set into a vacuum chamber of a sintering apparatus, which further comprises a temperature measurement device configured to determine the temperature of material in the die cavity during sintering. The apparatus further comprises a control system configured to move the temperature measurement device during sintering in order to maintain its relative positioning to a target location of the die cavity. The present invention further relates to a sintering apparatus capable of performing this method.

PRIORITY DATA

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/158,318, filed May 7, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and systems for spark plasma sintering, direct current sintering and hot pressing having a temperature measurement device moved during sintering.

2. Description of the Related Art

Spark plasma sintering (“SPS”), also referred to as direct current sintering (“DCS”) and field assisted sintering (“FAS”), is a pressure assisted high-speed powder consolidation/sintering technology related to hot-pressing (“HP”) capable of processing conductive and nonconductive materials. The mechanisms of DCS that provide rapid densification and material property enhancement are still under investigation. However the most commonly accepted mechanisms are rapid heating rates, joule heating of conductive powders and an electric field influence on densification.

During a typical DCS process, either an ON-OFF DC pulse (typically referred to as SPS) or a constant direct current is applied to a sample contained within a tooling material composed of graphite, metal, ceramic or a composite to generate joule heat. The heat is transferred to the sample by conduction, and, if the sample is conductive, electrical current can flow through it directly generating joule heat within the sample itself.

DCS's operational or “monitored” temperatures (200° C.-2400° C.) are commonly 200° C. to 500° C. lower than with conventional sintering, classifying DCS as a low-temperature sintering technology. Material processing (pressure and temperature rise and hold time) is typically completed in short periods of approximately five to twenty-five minutes. The relatively low temperatures combined with fast processing times ensure tight control over grain growth and microstructure enhancing material properties directly related to microstructure such as strength, toughness, electrical properties, thermal properties and optical properties.

Temperature during sintering is typically monitored using a pyrometer positioned at the sintering die. In order to ensure optimal sintering conditions, it is important to identify the hottest point on the sample being formed and to consistently monitor that position throughout the process. However, during sintering, compaction of the part occurs, leading to a need to dynamically reposition the pyrometer as the sintered product is formed. This is typically done manually, which is often difficult to do consistently and in a timely manner, as the optimum position changes constantly over the sintering time. Inaccuracies produce variability in product quality, especially run to run.

Thus, while spark plasma methods and systems are known, there is a need to provide for sintering systems and methods that improve process consistency both during and between cycles.

SUMMARY OF THE INVENTION

The present invention relates to a method of forming a sintered product. The method comprises the steps of loading a material to be sintered into a die cavity of a die set, which comprises a die casing and opposing rams configured to compress the material, and placing the die set into a vacuum chamber of a sintering apparatus. The sintering apparatus comprises a temperature measurement device configured to determine the temperature of material in the die cavity during sintering and further comprises a control system configured to move the temperature measurement device during sintering. In the present method, the temperature measurement device is positioned relative to a target location of the die cavity, and the material in the cavity is compressively sintered to form the sintered product. During the compressive sintering, the temperature measurement device is moved in order to maintain the relative positioning to the target location of the die cavity using the control system. The present invention further relates to a sintering apparatus capable of performing this method.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-4 show various elements of specific embodiments of the method and apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and system/apparatus for sintering materials under compression.

In general, a compressive sintering process, such as hot pressing, spark plasma sintering (SPS), and direct current sintering (DCS), involves the use of pressure and high temperature to convert a material to be sintered, especially in particulate or powder (fine particulate) form to a higher density product. SPS and DCS process are similar, with SPS using a pulsed direct current to generate heat and DCS using a non-pulsed direct current. Hot pressing and DCS differ in how and where the heat is generated. For example, DCS is a pressure assisted direct current heated sintering process that utilizes uniaxial force and direct current to consolidate powder material. Specifically, the application of DC voltage and current between powder material particles creates localized heating within a conductive powder compact and within the conductive die assembly (die set). Due to heat being generated within the die set and potentially within the powder, high heating rates are achievable, in contrast to conventional hot pressing where heat must be transferred into the die set from the exterior by radiant heating elements. During direct current sintering, heat is generated in and around the sample, rapidly heating it and limiting particle/grain growth due to the speed of the process. The entire process—from powder to finished bulk sample—is completed quickly, with high uniformity and without changing the particles' characteristics, specifically grain size and microstructure.

FIG. 1 illustrates a basic block diagram of a typical SPS/DCS apparatus, including the SPS/DCS system of the present invention. As shown, sintering apparatus 100 includes vacuum chamber 102 located within load frame 104 and further includes observation window 106 and temperature measurement device 108, both of which are incorporated into vacuum chamber 102. The material to be sintered (typically a powder material), is loaded into die set 111 and is placed within vacuum chamber 102 of sintering apparatus 100 wherein the process is performed. More specifically, as shown, die set 111 includes casing 112 and two opposing rams, lower ram 120 and upper ram 122, forming die cavity 110 in which the material to be sintered is placed. Sintering apparatus 100 further includes hydraulic power unit 116 and hydraulic press cylinder 118. The hydraulic power unit provides power to the hydraulic press cylinder, which in turn is used to move the lower ram and the upper ram up and down to manipulate the mechanical force (or pressure) applied, thereby compressing the sinterable material during the process. The force may be measured and monitored, such as with a load cell. In addition, a DC power supply 114 provides the necessary electrical current within vacuum chamber 102 during the compression. As shown, sintering apparatus 100 also includes vacuum pump 124, which allows the apparatus to operate under negative atmospheric pressure. Gas 126 can also be injected into vacuum chamber 102 if desired during the process. Central control system 128 can be used to control the different aspects of the sintering apparatus during use. For example, the control system can be used to control the DC power supply, the hydraulic power unit, the vacuum pump, as well as to control the amount of any inert gas introduced to the vacuum chamber during use. DCS systems are similar to SPS systems, with the use of a constant DC current applied to the material to be sintered.

In an DCS process, it is believed that the electrical current flowing between particles can assist in removing fine impurities and gases on and between the surfaces of the particles due to dielectric breakdown of surface oxides and local heating. In addition, the higher heating rates achievable allow the fine powders to be heated to high temperatures before grain coarsening can occur, allowing the powder to retain a high surface area to contribute to the sintering process, which progresses quickly

Force (pressure) also plays an important and predictable role in curbing particulate growth and influencing overall densities in SPS and DCS systems. For example, force multiplies diffusion throughout the sample as the material moves under pressure, especially during early sintering stages. Both too much and too little pressure can negatively influence the process. In large samples where high density is required, force is commonly increased in stages to enhance out-gassing at low temperature and sintering diffusion at higher temperatures. Accordingly, accurate manipulation of force can enhance the process.

For compression sintering systems, accurate determination of temperature is needed in order to monitor and control the process, thereby consistently producing sintered products with desired and predictable properties. As the material is consolidated and compressed, the temperature profile across the forming product changes. For example, the position of highest temperature across the material changes, and, as a result, the temperature measurement device, such as a pyrometer, must be repositioned during the sintering process in order to track and maintain monitoring of this position, in order to provide accurate process control. This is typically done manually, which is extremely difficult and time consuming to do accurately, especially between sintering runs. Thus, in the process and apparatus of the present invention, a control system is used to assess the relative position of the temperature measurement device to a target location of the die cavity of the die set, and, when the relative positioning differs from what is calculated, based on a predetermined algorithm, the control system causes the repositioning of the temperature measurement device, thereby maintaining the constant relative positioning.

A specific example is shown in FIG. 2, FIG. 3, and FIG. 4. However, it should be apparent to those skilled in the art that this is merely illustrative in nature and not limiting, being presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the present invention. In addition, those skilled in the art should appreciate that the specific conditions and configurations are exemplary and that actual conditions and configurations will depend on the specific system. Those skilled in the art will also be able to recognize and identify equivalents to the specific elements shown, using no more than routine experimentation.

As shown in FIG. 2, die set 211 comprises casing 212 and two opposing rams, lower ram 220 and upper ram 222, forming a die cavity in which the material to be sintered 205 (starting powder) is placed. As shown, the die cavity is vertically symmetrical, although other shapes are possible. A target location of the die cavity is identified, which, in this example, is the geometric centerline of the material. Assuming constant density throughout the sample, especially vertically, and heating, this would be the position of highest temperature. As can be seen in FIG. 2, as the starting powder is sintered to form the desired final sintered part 295, the position of the centerline changes. How this centerline moves during the sintering process can be predetermined and measured, and an example is shown in FIG. 3. As shown, the position of the centerline moves upward as the temperature of the material increases due to thermal expansion, and the increase depends on thermal properties of the specific material used. As sintering begins under compression, the position of the centerline drops as temperature increases and then begins to rise again as the sintered part also thermally expands.

It has been found that the response of this target location on the die cavity with temperature, which is related to the material within the die, can be used as an algorithm for a control system for changing the position of the compressive sintering system's temperature measurement device. Such a process is shown in FIG. 4. For example, a pyrometer is placed at a position relative to the die set to monitor the temperature of a target location of the die cavity, which, in this example, is the centerline of the die cavity. Once compressive sintering has begun, the position of the die cavity is determined, such as by accurately measuring the position of one or both of the rams, using a high resolution position measurement system. Then, the predetermined algorithm, accurately describing the position of the centerline with temperature, is used to calculate the desired position of the pyrometer, and a control system is activated to reposition the pyrometer in order to maintain the relative positioning to the target location of the die cavity, if needed. For example, a motorized stage can be used to reposition the pyrometer. Other devices or techniques will be known to one skilled in the art. This can be repositioning method can be repeated as often as needed throughout the compressive sintering process, either in steps (semi-continuously) or continuously. This automated process eliminates human error that occurs while adjusting the pyrometer position manually and improves process consistency by maintaining the same pyrometer position during each process cycle. In addition, the process and system, including a control system configured to move the temperature measurement device during sintering based on a predetermined algorithm relating position of a target location of the die cavity to temperature, can be adapted to a variety of different die configurations and materials, with appropriate changes included in the algorithm.

The foregoing description of preferred embodiments of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings, or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. 

What is claimed is:
 1. A method of forming a sintered product comprising, in any order, the steps: i) loading a material to be sintered into a die cavity of a die set comprising a casing and opposing rams configured to compress the material; ii) placing the die set into a vacuum chamber of a sintering apparatus, the sintering apparatus comprising a temperature measurement device configured to determine the temperature of material in the die cavity during sintering and a control system configured to move the temperature measurement device during sintering; iii) positioning the temperature measurement device relative to a target location of the die cavity; and iv) compressively sintering the material in the die cavity to form the sintered product, wherein, during the compressive sintering, the temperature measurement device is moved in order to maintain the relative positioning to the target location of the die cavity using the control system.
 2. The method of claim 1, wherein the method further comprises the step of determining an algorithm relating target location position to temperature, and wherein the control system moves the temperature measurement device based on the algorithm.
 3. The method of claim 3, wherein the sintering apparatus further comprises a repositioning device in communication with the temperature measurement device and the control system, and wherein the temperature measurement device is moved by the repositioning device using the control system.
 4. The method of claim 2, wherein the step of moving the temperature measurement device comprises determining a position of the die set during compressive sintering.
 5. The method of claim 4, wherein the die set comprises an outer casing, an upper ram, and a lower ram, and wherein the step of moving the temperature measurement device comprises determining a position of the upper ram, the lower ram, or both.
 6. The method of claim 1, wherein the temperature measurement device is moved continuously during the compressive sintering.
 7. The method of claim 1, wherein moving the temperature measurement device is automated.
 8. The method of claim 1, wherein the sintering apparatus is a spark plasma sintering apparatus or a direct current sintering apparatus.
 9. The method of claim 1, wherein the die cavity is vertically symmetrical.
 10. The method of claim 9, wherein the target location of the die cavity is a centerline.
 11. The method of claim 1, wherein the temperature measurement device is a pyrometer positioned on the vacuum chamber.
 12. A method of forming a sintered product comprising, in any order, the steps: i) loading a material to be sintered into a die cavity of a die set comprising an outer casing and an upper ram and a lower ram configured to compress the material; ii) placing the die set into a vacuum chamber of a sintering apparatus, the sintering apparatus comprising a pyrometer positioned on the vacuum chamber and configured to determine the temperature of material in the die cavity during sintering and a control system configured to move the pyrometer during sintering; iii) positioning the pyrometer relative to a centerline of the die cavity; and iv) compressively sintering the material in the die cavity to form the sintered product, wherein, during the compressive sintering, the pyrometer is moved in order to maintain the relative positioning to the centerline of the die cavity using the control system.
 13. A compressive sintering apparatus comprising a) a die set having a die cavity loaded with material to be sintered and comprising a casing and opposing rams configured to compress the material; b) a vacuum chamber into which the die set is placed c) a temperature measurement device configured to determine the temperature of material in the die cavity during sintering; and d) a control system configured to move the temperature measurement device during sintering based on a predetermined algorithm relating position of a target location of the die cavity to temperature. 